The present invention relates to a manhole, particularly to a manhole of which the lid is provided with a underside splayed rib ring and the ring seat is provided with a generally annular supporter having a bevelled underside. When the lid is vertically or diagonally placed over the ring seat, the splayed rib ring of the lid extends down beyond the ring seat and also the annular supporter of the ring seat. In the event the lid is lifted or removed in a non-vertical direction, the splayed rib ring is retained by the annular supporter.
Presently, there are various and numerous piping and drainage conduits set up underground and manholes for culverts of gas, running water, power cable, communication cable, military communication cable, underground drainage, etc. can be seen everywhere on the surface of the roads and bridges. However, these manholes are usually improperly designed and therefore, their lids frequently vibrate or bounce when they are run over by vehicles of high weight and/or travelling at very high speed. A vibrating or bounding manhole lid creates undesirable noise and can, sometimes, even bounce out of place to endanger nearby buildings, walkers, or vehicles, resulting in serious accidents and losses. In view of this, an improved manhole is needed to ensure safety.
The bounding out of place of a manhole lid may be attributable to the following causes:
1. The peripheries of currently used manhole lid 1a and ring seat 2a, as shown in FIGS. 13 and 14, are usually concentric and have too big a gap between them, while low manufacturing precision further enlarges this gap. This enlarged gap usually allows the vibration and bouncing of the manhole lid;
2. The rubber gasket 3a used for the known manhole lid 1a is usually provided at a place somewhat distant from the outer edge of the lid 1a and thereby, inevitably forms a fulcrum for pivoting thereof. Whenever one side of the outer edge of the lid 1a is subjected to a force, the opposite side thereof is raised due to leverage with the gasket 3a as the fulcrum. Due to its high speed, a vehicle may immediately run over the raised side. The impact of vehicle on the already raised side of a manhole lid 1a causes the first run over side to raise even higher. The wind pressure may cause a vacuum suction to form at the same time, as described in next paragraph, which further causes the lid 1a to bounce out of place;
3. A vacuum will form nearby the manhole lid 1a in the instance when a vehicle runs over the lid, which causes the pressure on the surface of lid 1a to be abruptly reduced and the lid 1a will raise because of the pressure imbalance below and above the lid 1a. At the same time, surrounding air with higher density flows toward the point being run over by the vehicle which causes an increase in wind pressure. The faster the vehicle speed, the stronger the wind pressure and the suction vacuum. The wind pressure and the suction force will move forward following the speedy movement of the vehicle. All of these forces form a resultant force in normal direction that lifts the manhole lid 1a; and,
4. Since the peripheries of the lid 1a and the ring seat 2a are usually concentric with each other and have the same gradiant in the same direction, no resistance forms when the lid 1a is raised from the ring seat 2a at the time it is run over by a vehicle. This situation, plus the above mentioned state which forms an upward suction and the wind pressure caused by surrounding air flowing toward the lid 1a, further helps the lid 1a to bounce and even fly out of place.
In addition to the noise and possible damages caused by a bouncing and even flying out of the manhole lid, there are two other disadvantages found in existing manholes:
1. Since the quality of an existing manhole is largely dependent on the manufacturing precision of the present device, it is necessary to inspect and accept the manholes as a set, that is, a lid 1a must be used together with its matched ring seat 2a and cannot be conveniently replaced by other ones. This obviously causes much unnecessary trouble in practical use, maintenance, transportation, and storage of manholes. To replace the original lid 1a with another lid might very possibly lose the match of the lid 1a with the ring seat 2a and thereby worsen the bounce situation.
2. Furthermore, most of the existing manholes are made of cast metal which is electrically conductive. When the cables under the manhole become worn out, a leakage might occur when the conductive wire in the cable contacts the metal lid 1a or ring seat 2a. For a power cable of high voltage, explosion or shock might cause serious injury, damage, loss, or even death.
There is a need for a manhole with a diagonally retained lid, which is simply structured, easily manipulable, requires a lower-precision in manufacture, and which can prevent the manhole lid from bouncing out of place, emitting noise, and causing leakage.